This disclosure relates generally to an electromagnetic tracking system that uses electromagnetic fields to determine the position and orientation of an object, and more particularly to an asymmetrical coil arrangement for an electromagnetic tracking system.
Electromagnetic tracking systems have been used in various industries and applications to provide position and orientation information relating to objects. For example, electromagnetic tracking systems may be useful in aviation applications, motion sensing applications, retail applications, and medical applications. In medical applications, electromagnetic tracking systems have been used to provide an operator (e.g., a physician, surgeon, or other medical practitioner) with information to assist in the precise and rapid positioning of a medical device, implant or instrument located in or near a patient's body during image-guided surgery. An electromagnetic tracking system provides positioning and orientation information for a medical device, implant or instrument with respect to the patient or a reference coordinate system. An electromagnetic tracking system provides intraoperative tracking of the precise location of a medical device, implant or instrument in relation to multidimensional images of a patient's anatomy.
An electromagnetic tracking system uses visualization tools to provide a medical practitioner with co-registered views of a graphical representation of the medical device, implant or instrument with pre-operative or intraoperative images of the patient's anatomy. In other words, an electromagnetic tracking system allows a medical practitioner to visualize the patient's anatomy and track the position and orientation of a medical device, implant or instrument with respect to the patient's anatomy. As the medical device, implant or instrument is positioned with respect to the patient's anatomy, the displayed image is continuously updated to reflect the real-time position and orientation of the medical device, implant or instrument. The combination of the image and the representation of the tracked medical device, implant or instrument provide position and orientation information that allows a medical practitioner to manipulate a medical device, implant or instrument to a desired location with an accurate position and orientation.
Generally, an electromagnetic tracking system may include an electromagnetic transmitter with an array of one or more transmitter coils, an electromagnetic receiver with an array of one or more receiver coils, electronics to generate a current drive signal for the one or more transmitter coils and to measure the mutual inductances between transmitter and receiver coils, and a computer to calculate the position and orientation of the receiver coil array with the respect to the transmitter coil array, or vice versa. An alternating current drive signal is provided to each coil of the electromagnetic transmitter, generating an electromagnetic field being emitted from each coil of the electromagnetic transmitter. The electromagnetic field generated by each coil in the electromagnetic transmitter induces a voltage in each coil of the electromagnetic receiver. These voltages are indicative of the mutual inductances between the coils of the electromagnetic transmitter and the coils of the electromagnetic receiver. These voltages and mutual inductances are sent to a computer for processing. The computer uses these measured voltages and mutual inductances to calculate the position and orientation of the coils of the electromagnetic transmitter relative to the coils of the electromagnetic receiver, or the coils of the electromagnetic receiver relative to the coils of the electromagnetic transmitter, including six degrees of freedom (x, y, and z measurements, as well as roll, pitch and yaw angles).
Electromagnetic tracking systems may be limited by the number of degrees of freedom they are able to track. In general, the number of degrees of freedom that an electromagnetic tracking system is able to track and resolve depends on the number of transmitting and receiving coils in the system. For example, a system comprising a single transmitting coil and multiple receiver coils may track a device or instrument in only five degrees of freedom (x, y, and z coordinates, as well as pitch and yaw angles). The roll angle is not measurable. As will be appreciated, the magnetic field from a coil small enough to be approximated as a dipole is symmetrical about the axis of the coil (coil's roll axis). As a result, rotating the coil about the coil's axis (i.e., the degree of freedom commonly known as “roll”) does not change the magnetic field. The processor performing the processing cannot resolve the rotational orientation (roll) of the coil. Consequently, only five degrees of freedom of position and orientation are trackable.
One approach of obtaining the roll angle measurement is to add another coil to the electromagnetic transmitter or electromagnetic receiver configuration. However, having two coils in close proximity may introduce “mutual inductance coupling” into the mix. Mutual inductance coupling between coils can negatively impact accuracy performance of an electromagnetic tracking system because cross-coupling currents cannot be accurately measured. Mutual inductance coupling between two coils permits the current in one coil to induce a voltage in the second coil, causing current flow in the second coil with the first coil's waveform. This unwanted current makes distinguishing the two coils' magnetic fields more difficult.
Therefore, there is a need for a coil arrangement for an electromagnetic transmitter or receiver assembly in an electromagnetic tracking system that provides maximum tracking accuracy and the ability to measure six degrees of freedom of the position and orientation of a medical device, implant or instrument.